Polymer nanocomposites are composite materials containing at least one phase that has nano-scale dimensions (Winey, K. I. et al. MRS Bulletin 2007, 32, 314-322). Compared to microcomposites, nanocomposites have much lower volume additions of the nanoparticles, which facilitate processing and result in lighter products. In order to achieve significant improvements in properties of the composite (such as the mechanical properties, barrier properties, thermal stability, flame retardancy or chemical resistance of polymer nanocomposites), the nanoparticles have to be compatible with the polymer matrices and dispersed well in the polymer matrices to ideally produce exfoliated structures.
Nanocrystalline cellulose (NCC), a chiral nematic structure of high strength and elastic modulus, is renewable and possesses a relatively large reactive surface. The major obstacles to NCC application in composite manufacture are: (1) aggregation of NCC particles, (2) poor dispersion of the hydrophilic NCC particles in mostly hydrophobic polymer matrices, and (3) poor interfacial adhesion between NCC and polymer. Different approaches have been followed to increase NCC's dispersion and interaction with polymer matrices. NCC has been either coated with surfactant or chemically surface modified (see Samir, M. et al. Biomacromolecules, 2005, 6 (2), 612-626 and Lima, M. M. D. et al. Macromolecular Rapid Communications, 2004, 25 (7), 771-787). Use of surfactants is a simple enough method, but a large amount of surfactant is normally required which would negatively impact the strength of the resulting composite. Surface modification, on the other hand, generally involves reaction with the hydroxyl groups on the NCC surface. Silanes have, for example, been employed to graft hydrophobic groups onto the NCC surface. Moreover, polymers with hydroxyl reactive groups have been used as well, such as polyethylene glycol (PEG) (see Araki, J. et al. Langmuir, 2001, 17 (1), 21-27, Polycaprolactone (PCL) (see Habibi, Y. et al. Biomacromolecules, 2008, 9 (7), 1974-1980) and poly(propylene) (PP) (see Ljungberg, N. et al. Biomacromolecules 2005, 6 (5), 2732-2739). Such modifications can make NCC more hydrophobic and give NCC reasonable stability in organic solvents. However, these reactions (i) generally involve several, intricate steps, (ii) are therefore costly, and (iii) have limited scalability.